The uses of optical fiber devices are increasing for communications and sensing applications due to inherent advantages in bandwidth, size, weight, immunity to electromagnetic interference, and ruggedness. The conditions to which these devices are exposed necessitate packaging of the components in a way that will withstand various environmental effects. For communications applications, this means isolating the device from the environment in a way such that the operation of the device is not altered by peturbations such as temperature and bending. In sensing applications, it is desirable that the packaging of the optical sensor allows the sensor to be exposed to one specific isolated perturbation without exposing the sensor to other environmental factors such as bending or strain.
Traditionally, optical fiber devices used in communications applications shield the device entirely from the environment. The optical fiber is packaged by attaching the device to a substrate made from a low thermal expansion material. Typically, the material chosen is based on closely matching the thermal properties of the material to that of fused silica. Various epoxies that have a low thermal response are used in the attachment procedure. The device is then encased in a secondary epoxy or material that isolates the optical fiber device from strain and outside environmental effects. The coating also prevents material from encountering the optical fiber device. A protective sheath is used for additional strength and protection around the outside of the component. In some cases, the fiber that ingresses and egresses the packaged device is protected with additional sheathing that acts as a strain relief. This method results in the component being completely shielded from the environment and is not useful for applications requiring exposure to a particular environment.
Bulk optic devices or extrinsic components are also packaged to isolate response from the environment. The methods of isolation include hard mounting the bulk components and isolating alignment from the environment through the use of ceramic ferrules. As with the communications applications, no external parameter is allowed to influence the performance of the optical fiber device and thus this packaging is inadequate for applications requiring direct environmental contact.
Optical fibers employed in sensing applications require similar packaging considerations to those used for communication and bulk optic devices. The fiber Bragg grating (FBG) is one of the most deployed optical fiber sensors and produces a spectrally dependent signal. Changes in the environment shift the operational wavelength of the device. Measurement of the wavelength provides an indication of perturbation strength. Typically, FBG devices have been used to monitor strain or temperature. Packaging requires a strain compatibility with the sensor to realize actual strains present in the surroundings. A strong strain transfer is accomplished by using an epoxy to attach the fiber device in a small thin walled steel tube. The fiber is jacketed with cabling to provide enhanced survivability. This packaging works well for measuring strain, however, it cannot be used for applications where the strain and bending factors are to be minimized or eliminated.
In other packaging methods, the fiber device is directly attached to the test surface and an epoxy is used as an overcoat; or the FBG sensors are discretely attached to surfaces using localized epoxy sites. This attachment allows the sensor to measure environmental changes such as strain and bending but, due to strain transfer, does not allow for the detection of other isolated environmental factors such as temperature or refractive index changes.
Flow cells have been constructed for liquid-phase measurements using fluorescent-based devices that require the sensor surface be in contact with the environment. These flow cells are primarily designed to enhance the sensing characteristics of a particular component by blocking background light from influencing sensor response. The device enclosure is constructed in a way to limit background light, a primary noise factor in fluorescent applications. The cell does not take into account rigid support for the optical fiber or additional processing needs such as mode stripping. Lastly, flow cells have limiting configurations that require external pumps or other methods to bring the external environment to the sensor as opposed to directly exposing the sensor to the external environment.